Phosphate esters and their use as adhesive promoters

ABSTRACT

A novel class of phosphate esters having a formula comprising either:   WHEREIN R is selected from the group consisting of   AN UNSATURATED HYDROCARBON RADICAL,   D R A W I N G

United States Patent [191 de Majistre et al.

[451 Aug. 28, 1973 PHOSPHATE ESTERS AND THEIR USE AS ADHESIVE PROMOTERS[75] Inventors: Robert de Mailstre, Natrona Heights; Gordon M. Parker,Lower Burrell; Robert Slrkoch, Pittsburgh, all of Pa.

[73] Assignee: PPG Industries, Inc., Pittsburgh, Pa.

[22] Filed: Mar. 29, 1971 [21] Appl. No.: 129,113

[52] US. Cl. 117/72, 117/93.31, 117/161 UB, 117/161 UC, 117/161 K,204/159.22, 260/75 [51] Int. Cl 844d l/50 [58] Field of Search117/93.31, 161 K, 117/161 UC, 161 UB, 72; 260/8955, 75 P,

[56] References Cited UNITED STATES PATENTS 3,346,545 10/1967 Sehm260/952 3,099,676 7/1963 Lanham i 260/928 3,361,842 1/1968 Applegata etal.... 1l7/93.31 3,455,801 7/1969 DAlelio 117/93.3l

Primary ExaminerAlfred L. Leavitt Assistant Examiner.lohn H. NewsomeAttorney-Chisholm & Spencer [57] ABSTRACT A novel class of phosphateesters having a formula comprising either:

wherein R is selected from the group consisting of an unsaturatedhydrocarbon radical,

wherein R is an alkylene radical, R is an alkyl radical or hydrogen, Ris an alkyl radical or hydrogen, R is an alkyl radical or hydrogen, andR is a trivalent organic moiety, and R is selected from the groupconsisting of an alkyl radical and i RgORq wherein R is R,OR,, or R andR has the formula:

wherein R is R,[OR,+,,, R,,[OR,lor R (COOR,)- R is alkylene, arylene oraralkylene, p is from O to 30, y is from 0 to 20, n is either 1 or 2, Liseither00r1,andn+m+L=3,bis3to5,andz is from 2 to 5, and t is from 0 to5; is useful as an adhesion promoter for substrates coated with actiniclightsensitive coatings and ionizing irradiation-sensitive coatings,

10 Claims, No Drawings A method of forming coatings, having goodstrength, wear resistance, stain resistance, and other propertiesachievable only by extensive crosslinking of the coating material isthat of coating a substrate with an ionizing radiation-sensitive coatingor an actinic light-sensitive material such as polyacrylates andunsaturated polyester resins and subjecting the coatings to ionizingirradiation or actinic light to cure. This method of coating substratesis advantageous as the degree of crosslinking achieved thereby isunobtainable by any other method. It has been found, however, that thesecoating compositions are somewhat difficult to adhere to some substratessuch as metal.

A novel class of phosphate esters has now been discovered which may beused as a pretreatment of substrates to enhance the adhesion of ionizingradiationsensitive or actinic light-sensitive materials thereto or as anadditive to the coating materials to increase the adhesion of thecoatings to the substrates.

The novel phosphate esters of this invention have a formula selectedfrom the group consisting of:

wherein R is selected from the group consisting of:

I CH:=CCOOR R1, an

unsaturated hydrocarbon radical,

and R is R,0R, or R, and n is either 1 or 2, m is either l or 2, L iseitherO or 1, and n+m +L= 3, and

(B) ROLLOLI,

wherein R is as described above, R, has the formula:

wherein R is R,[OR,-]-,,, R [OR,-},,, or R (CO0R,+,,, R, is alkylene,arylene, or aralkylene, p is from 0 to 30, y is from 0 to 20, b is 3 to5, and z is from 2 to 5, and

OH H H wherein R and R are as described above and t is from The radicalR may be or an unsaturated hydrocarbon radical. The alkylene radical maybe any unsaturated hydrocarbon such as vinyl, allyl, butylenyl and thelike. Preferably, the unsaturated hydrocarbon radical contains up toabout 13 carbon atoms.

R, may be any alkylene such as methylene, ethylene, propylene, butylene,heptylene and the like, and preferably contains from two to four carbonatoms. It is noted that where more than one R, radical is present, theR, radicals may be different species of the group R,.

R R and R may be hydrogen or an alkyl radical such as methyl, ethyl,butyl, octyl, decyl, lauryl, and the like. Generally, it is preferredthat the alkyl contain from one to about 20 carbon atoms. Of course, R Rand R may be the same or different radicals, that is, when R, is ethyl,R, may be hydrogen and R may be isopropyl.

R is a trivalent organic moiety which is preferably a saturatedhydrocarbon such as and the like. It is preferred that R, contains fromabout 2 to about 7 carbon atoms when it is a saturated hydrocarbonmoiety.

The radical R is either RiORr or an alkyl radical wherein R is R, orR,OR,. The preferred alkyl radicals for R contain up to about 40 carbonatoms such as ethyl, butyl, hexyl, and the like.

The radical R, has the formula F? i I 424:0 CRnCORswherein R is-R,[OR,+,,, R [OR,+,, or R (COOR)-,, and wherein y is from to 20 and Ris either an alkylene radical such as ethylene, propylene, isobutylene,octylene, and the like, an arylene radical such as phenylene,naphthylene, tolylene, and the like, or an aralkylene radical such asphenyl ethylene, phenyl propylene, and the like. It is preferred that Rcontain from about two to about 30 carbon atoms. p is from 0 to about30.

The novel phosphate esters of this invention are prepared by reactingpolyphosphoric acid or P 0 with a hydroxy-containing ethylenicallyunsaturated monomer which is cross-linkable with either actinic light orionizing irradiation. A hydroxy-containing saturated ornon-ethylenically unsaturated moiety such as butanol, bis-hydroxyethylmaleate, bis-hydroxy-ethyl azelate, and the like, may also be reactedwith the above reactants. The hydroxy may be primary, secondary ortertiary.

The reaction may be carried out at temperatures from about 20 to about140C. and requires no catalyst.

The anhydride equivalent ratio of polyphosphoric acid or P 0 to thehydroxy-containing ethylenically unsaturated monomer is from about 1:1to about 1:15. The preferred ratio is about 1:1. It is pointed out thatless than 1:1 ratio leaves unreacted phosphate anhydride functionalitywhich is undesirable. The hydroxycontaining saturated moiety maycomprise up to about two times the molar amount of thehydroxy-containing ethylenically unsaturated monomer.

The above prepared phosphate esters are preferably used as pretreatmentsfor substrates to be coated with actinic light-sensitive or ionizingirradiation-sensitive coatings.

Although any substrate material may be pretreated and coated as outlinedabove, such as metal, plastic, wood, cloth, and the like, because of thetremendous difficulty inherent in adhering radiation-sensitive coatingsto metals, it is preferred that the substrate be a metal, such asaluminum or steel.

The actinic light-sensitive and ionizing irradiationsensitive coatingmaterials may be any radiation curable organic materials. The mostuseful radiationsensitive organic materials are polyester resins andacrylic resins.

The polyester resins comprise unsaturated polyesters, solubilized invinyl monomers. The unsaturated polyesters are ordinarily mixtures ofalpha-beta ethylenically unsaturated polycarboxylic acids and polyhydricalcohols.

The ethylenically unsaturated polycarboxylic acids include such acidsas:

maleic acid fumaric acid aconitic acid mesaconic acid citraconic aciditaconic acid and halo and alkyl derivatives of such acids and the like;the preferred acid being maleic acid. The anhydrides of these acids,where the anhydrides exist, are, of course, embraced under the termacid, since the polyesters obtained therefrom are essentially the samewhether the acid or anhydride is utilized in the reaction. Theethylenically unsaturated dicarboxylic acids are conventionally employedin an amount of about 10 mol percent to about 100 mol percent, althoughpreferably in an amount of about 20 mol percent to about mol percent ofthe total mols of acid component in the polyester.

The polyhydric alcohols useful in preparing unsaturated polyestersinclude:

ethylene glycol diethylene glycol triethylene glycol polyethylene glycolpropylene glycol dipropylene glycol polypropylene glycol glycerolneopentyl glycol pentaerythritol trimethylol propane trimethylol ethaneand the like. The preferred polyols for the purposes of this inventionhave a molecular weight of less than about 2,000 and consist essentiallyof carbon, hydrogen and oxygen. The poly-hydric alcohols are generallyemployed in an equal molar-ratio to the total acid components, or as aslight excess, as, for example, about 5 mol percent excess.

Saturated dicarboxylic acids may be utilized in combination with theunsaturated acid or anhydride in the preparation of unsaturatedpolyesters. Such acids increase the length of the polyester withoutadding additional cross-linking sites, which is a desired feature insome polyesters. Examples of useful dicarboxylic acids which are eithersaturated or only aromatically unsaturated include:

Succinic acid adipic acid suberic acid azelaic acid sebacic acidisophthalic acid terephthalic acid tetrachlorophthalic acid and thelike. As in the case of the ethylenically unsaturated acids, theanhydrides of these acids, where the anhydrides exist, are, of course,embraced in the term acid," since the polyesters obtained therefrom arethe same. Furthermore, for purposes of the present invention, thearomatic nuclei of such acids as phthalic acid are generally regarded assaturated since the double bonds do not react by addition, as doethylenic groups. Therefore, whereever the term saturated dicarboxylicacid is utilized, it is to be understood that such term includes thearomatically unsaturated dicarboxylic acids. Such saturated carboxylicacids" may also be referred to as non-olefinically unsaturatedpolycarboxylic acids.

Vinyl monomers which crosslink with unsaturated polyesters to formthermosetting materials may be interpolymerized with the acryliccompounds and polyesters, if desired. Such vinyl monomers may include:

styrene alphwmethylstyrene divinylbenzene diallyl phthalate methylacrylate methyl methacrylate hexyl acrylate octyl acrylate octylmethacrylate diallyl itaconate diallyl maleate and the like. Thepreferred vinyl monomers are liquid compounds, soluble in the polyestercomponents. Such monomers should preferably be free of non-aromaticcarbon carbon conjugated double bonds.

The vinyl monomer as exemplified in the above list may be employed overa broad range, but usually the proportion thereof, upon a weight basis,will be less than the polyester component. The amount of monomer shouldbe sufficient to provide a liquid, flowable, interpolymerizable mixture.Ordinarily, the percentage of monomer will fall within the range ofabout percent to about 60 percent by weight of the total mixture ofpolyester and monomer.

The preferred polyester resins are those formed from polyesters ofpropylene glycol or neopentyl glycol as the diol and maleic acid andisophthalic acid as the carboxylic acids with styrene or diallylphthalate or vinyl toluene as the solubilizing monomer.

The acrylic compositions which may be used as the radiation-sensitivematerials in this invention may be hydroxy-containing esters or amidesof acrylic or methacrylic acid or co-monomers of such an ester withanother copolymerizable monomer. Suitable esters include those ofalcohols containing one to eight carbon atoms such as hydroxy methylacrylate, hydroxy methyl methacrylate, hydroxy ethyl acrylate, hydroxybutyl methacrylate, hydroxy octyl acrylate, and hydroxy 2- ethoxy ethylmethacrylate. Suitable amides include hydroxy acrylamide, hydroxymethacrylamide, hudroxy tertiary butyl acrylamide and primary hydroxyalkyl acrylamides. Mixtures of such esters or amides may becopolymerized or one or more of the esters may be copolymerized with ahigher alkyl ester or amide of acrylic or methacrylic acid or withanother monomer containing a copolymerizable vinyl group, for example,itaconate esters, maleate esters, and allyl compounds. Alkylenedimethacrylates and diacrylates, such as 1,3- butylene dimethacrylate,and the like, and triacrylates and trimethacrylates, such as trimethylpropane trimethacrylate, and the like, may also be used. The preferredacrylates are polyacrylates and methacrylates, such as diacrylates,dimethacrylates, triacrylates, trimethacrylates, and the like, such asacryloxy pivalyl acryloxy pivalate,bis-(acryloxyethyl)-hexahydrophthalate and polymers thereof,bis-(acryloxyethyl) phthalate and polymers thereof, and the like.Examples of these materials are found in U. S. Pat. No. 3,455,802 andBritish Pats. Nos. 1,162,722 and 1,162,721. The preferred materials tobe reacted with 1 ,0 or polyphosphoric acid are hydroxy ethyl acrylateand allyl alcohol.

The pretreatment of the substrate with the phosphate ester comprisesapplying a thin film of up to about 0.1 mil to the substrate by making asolution of about 5 to about percent by weight of the ester in a lowboiling solvent like acetone and drying the solvent.

The term irradiation, as used herein, means high energy radiation and/orthe secondary energies resulting from conversion of elctrons or otherparticle energy to X-rays or gamma radiation. While various types ofirradiation are suitable for this purpose, such as X-ray and gamma rays,the radiation produced by accelerated high energy electrons has beenfound to be very conveniently and economically applicable and to givevery satisfactory results. However, regardless of the type of radiationand the type of equipment used for its generation or application, theuse thereof in the practice of the invention as described herein iscontemplated as falling within the scope of this invention so long asthe ionization radiation is equivalent to at least about 20,000 electronvolts.

While there is no upper limit to the electron energy that can be soapplied advantageously, the effects desired in the practice of thisinvention can be accomplished without having to go to above about20,000,000 electron volts. Generally, the higher the electron energyused, the greater is the depth of penetration into the massive structureof the materials to be treated. For other types of radiation, such asgamma and X-rays, energy systems comparable to the above range ofelectron volts are desirable.

It is intended that the term irradiation" include what has been referredto in the prior art as ionizing radiation which has been defined asradiation possessing an energy at least sufficient to produce ions or tobreak chemical bonds and thus includes also radiations such as ionizingparticle radiation as well as radiations of the type termed ionizingelectromagnetic radiation.

The term ionizing particle radiation has been used to designate theemission of electrons or highly accelerated nuclear particles such asprotons, neutrons, alpaparticles, deuterons, beta-particles, or theiranalogs, directed in such a way that the particle is projected into themass to be irradiated. Charged partilces can be accelerated by the aidof voltage gradients by such devices as accelerators with resonancechambers, Van der Graaff generators, betatrons, synchrotons, cyclotrons,etc. Neutron radiation can be produced by bombarding a selected lightmetal such as beryllium with positive particles of high energy. Particleradiation can also be obtained by the use of an atomic pile, radioactiveisotopes or other natural or synthetic radioactive materials.

Ionizing electromagnetic irradiation is produced when a metallic target,such as tungsten, is bombarded with electrons of sutiable energy. Thisenergy is conferred to the electrons by potential accelerators of over0.1 million electron volts (mev.). In addition to irradiation of thistype, commonly called X-ray, an ionizing electromagnetic irradationsuitable for the practice of this invention can be obtained by means ofa nuclear reactor (pile) or by the use of natural or syntheticradioactive material, for example, cobalt 60.

Various types of high power electron linear accelerators arecommercially available, for example, the ARCO type travelling waveaccelerator, model Mark 1, operating at 3 to 10 million electron volts,such as supplied by High Voltage Engineering Corporation, Burlington,Massachusetts, or other types of accelerators as described in US. Pat.No. 2,763,609 and in British Pat. No. 762,953 are satisfactory for thepractice of this invention.

The term actinic light, in general, refers to the use of wave lengths ofapproximately 1,800 to 4,000 angstrom units. Various suitable sources ofthe actinic light are available in the art including, by way of example,mercury lamps, black lights, ultraviolet cored carbon arcs, andhigh-flash lamps.

1f actinic light is used to cure the material, it is desirable to addphotosensitizers such as benzoin, benzoin methyl ether, diphenyldisulfide, dibenzyl disulfide, benzyl, and the like. Generally, thematerial may comprise from about 0.1 percent by weight of thephotosensitizers to about percent by weight of the photosensitizers.

The phosphate esters may also be cured by any free radical mechanismsuch as peroxide cures. In this case, it is preferable to cure in theabsence of air.

As an alternative method of adhering the coatings to the substrate, thephosphate esters may be used as an additive to the coating materials tobe used. The materials may be mixed without the use of heat or catalystand generally the mixture should contain at least about 0.1 percent byweight of the phosphate ester.

The coated substrates produced by the methods of this invention may beused as conduits, interior coated metal articles such as filingcabinets, exterior coated articles such as aluminum siding, and thelike.

The following examples set forth specific embodiments of the invention.However, the invention is not to be construed as being limited to theseembodiments for there are, of course, numerous possible variations andmodifications. All parts and percentages of the examples as well asthroughout the specification are by weight unless otherwise indicated.

EXAMPLES 1-3 A reactor was charged with 371 grams of 2- hydroxyethylacrylate and 1 gram of hydroquinone and heated to 50C. Over a 1-hourperiod, 142 grams of P 0 were added at temperatures from 65C. to 70C.

The final product had an acid value of 305.26 and a mixture of materialshaving the structures:

An ionizing-irradiation sensitive coating material was tested foradhesion to various substrates both with and without the addition of thenovel phosphate ester of this invention. In each case the phosphateester above was used in different amounts with acryloxy pivalyl,acryloxy pivalate.

Different substrates were used for the tests and the coatings were curedby subjecting to electron beam bombardment at 500 kilovolts to a totaldosage of 3 megarads in a nitrogen atmosphere. The coated panels weretested for adhesion by the Tape Test which consists of bumping thepanels with a force of 20,40 and 60-inch pounds and then scratching an Xshape on the panel with a knife and pressing cellophane tape on to thearea and then peeling off. The adhesion of the coating is measured bythe amount of coating peeled off with the tape.

The results were as follows in Table 1:

Product of Ex. Phospahte Substrate Adhesion Ester A 0 Dipped galvanizedtaped ofi' steel 1 0.2 Dipped galvanized slight tape steel off B 0Pickled steel taped off 2 0.2 Pickled steel taped off C 0 Aluminum tapedoff 3 0.2 Aluminum no tape off As seen from the results above, theradiationsensitive materials when combined with the novel phosphateester of this invention have much improved adhesion over the materialsalone.

EXAMPLES 4-6 Bis-(acryloxyethyl) hexahydrophthalate was tested foradhesion to various substrates both with and without the addition of thenovel phosphate ester of Example l.

Tests similar to that of Examples 1-3 were run to determine the adhesionof the materials to the various substrates. The following results wereobtained:

Product of Ex. Phosphate Substrate Adhesion Ester D 0 Dipped galvanizedtaped off steel 4 5.0 Dipped galvanized no tape off steel E 0 Pickledsteel taped off 5 5.0 Pickled steel no tape off F 0 Aluminum taped off 65.0 Aluminum no tape off EXAMPLES 7-9 Bis-(acryloxyethyl) phthalate wastested for adhesion to various substrates both with and without theaddition of the novel phosphate ester of Example 1.

Tests similar to those of Examples 1-6 were run to determine theadhesion of the materials to the various substrates. The followingresults were obtained:

Product of Ex. Phosphate Substrate Adhesion Ester G 0 Dipped galvanizedtaped off steel 7 7.0 Dipped galvanized no tape off steel H 0 Pickledsteel taped off 8 7.0 Pickled steel no tape 05 l 0 Aluminum taped off 97.0 Aluminum no tape off EXAMPLES 10-12 Bis-(acryloxyneopentyl) adipatewas tested for adhesion to various substrates both with and without theaddition of the novel phosphate ester of Example 1.

Tests similar to that of Examples 1-9 were run to determine the adhesionof the materials to the various substrates. The following results wereobtained:

Product of Ex. Phosphate Substrate Adhesion Ester J 0 Dipped galvanizedtaped off steel 10 1.2 Dipped galvanized no tape off steel K 0 Pickledsteel taped off 11 1.2 Pickled steel taped off L 0 Aluminum taped off 121.2 Aluminum no tape off EXAMPLE 13 The effect of a pretreatment withthe novel phosphate esters of this invention on adhesion was tested bycoating an untreated substrate with a radiationsensitive coating and atreated substrate with the same coating and comparing the adhesion.

An aluminum panel and tin-free steel panel were treated by dipping thepanels into a 10 percent solution of the phosphate ester of Example 1 inacetone and dried at F. in an oven. A 0.6 mil drawdown coating ofbis-(acryloxyethyl) hexahydrophthalate was made on both the abovetreated panels and two untreated panels and cured by subjecting toelectron beam impingement to a total dosage of 3 megarads in nitrogen.

The untreated panels showed very poor adhesion while the treated panelshad outstanding adhesion.

According to the provisions of the patent statutes, there is describedabove the invention and what are now considered to be its bestembodiments. However, within the scope of the appended claims, it is tobe understood that the invention can be practiced otherwise than asspecifically described.

We claim:

1. The method of forming an adherent coating on a substrate comprisingmixing an ionizing irradiationsensitive coating material with a compoundhaving the formula selected from the group consisting of wherein R isselected from the group consisting of H capo-0001110111, oH,= :oooR10Ri.

an unsaturated hydrocarbon radical,

wherein R, is R OR or R and n is either 1 or 2, m is either 1 or 2, L iseitherO or 1, and N+M+L=3,

An J.

wherein R is as above, R, has the formula ll ll :1 Rs OCNHR NI-ICOR! vwherein R is as above, R, is alkylene, arylene, or aralkylene, p is fromo to 30, y is from to 20, b is from 3 to 5, and z is from 2 to 5, and

OH H t OH wherein R and R are as above and t is from 0 to 5, andapplying to the substrate and subjecting the coated substrate toionizing irradiation.

2. The method of claim 1 wherein the coating is sub- 5 jected to a totaldosage of from about 0.2 megarad to about 300 megarads.

3. The method of claim 1 wherein the coating material is acryloxypivalyl, acryloxy pivalate.

4. The method of claim 1 whrein the coating material isbis-(acryloxyetbyl) hexahydrophthalate.

5. The method of claim 1 wherein the coating material isbis-(acryloxyethyl) phthalate.

6. The method of forming an adherent coating on a substrate comprisingapplying to the substrate a compound having the formula selected fromthe group consisting of H (13H: CHg=-C 0 O B1011 CHz=C-" O 1 1.

an unsaturated hydrocarbon radical,

wherein R is an alkylene radical, R is an alkyl radical or hydrogen, Ris an alkyl radical or hydrogen, R is an alkyl radical or hydrogen, andR, is a trivalent organic moiety, and R is selected from the groupconsisting of an alkyl radical and wherein R is R OR or R and n iseither 1 or 2, m is either 1 or 2, L is either 0 or 1, and n m L 3, and

wherein R is as above, R has the formula wherein R is as above, R isalkylene, arylene, or aralkylene, p is from 0 to 30, y is from 0 to 20,b is from 3 to 5, and z is from 2 to 5, and

wherein R and R are as above and l is from to 5, and 8. The method ofclaim 6 wherein the coating matesubsequently applying an ionizingirradiation-sensitive rial is acryloxy pivalyl, acryloxy pivalate.coating material to the substrate and subjecting the 9. The method ofclaim 6, wherein the coating matecoated substrate to ionizingirradiation. rial is bis-(acryloxyethyl) hexahydrophthalate.

7. The method of claim 6 wherein the coating is sub- 5 10. The method ofclaim 6 wherein the coating matejected to a total dosage of from about0.2 megarad to rial is bis-(acryloxyethyl) phthalate. about 300magarads.

Patent No. 3,754,972 Dated August 28 7 Inventor(s) Robert DeMajistre eta1 It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

First inventors name "Robert de Majistre" should be -Robert DeNajistre-.

Claim 1, line 60, "R [OR +y, I R [OR should be R OR R [OR Claim 4,"whrein" should be 'wherein.

- O a H H II Claim 6, line 55, "R -[OCNHR NHCOR ]p" should be '.-.R[OCNHR O i! Nnc0R line 59, "R [OR +y, R [OR should be R [0R R mR Signedand sealed this 25th day of December 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. RENE D. TEGTMEYER Attest'ing Officer ActingCommissioner of Patents FORM PO-VJSO 10-69) USCOMMDC 60376P59 U75.GOVERNMENT PRINTINQ OFFIPE I969 0"3563Jl,

2. The method of claim 1 wherein the coating is subjected to a totaldosage of from about 0.2 megarad to about 300 megarads.
 3. The method ofclaim 1 wherein the coating material is acryloxy pivalyl, acryloxypivalate.
 4. The method of claim 1 whrein the coating material isbis-(acryloxyethyl) hexahydrophthalate.
 5. The method of claim 1 whereinthe coating material is bis-(acryloxyethyl) phthalate.
 6. The method offorming an adherent coating on a substrate comprising applying to thesubstrate a compound having the formula selected from the groupconsisting of
 7. The method of claim 6 wherein the coating is subjectedto a total dosage of from about 0.2 megarad to about 300 magarads. 8.The method of claim 6 wherein the coating material is acryloxy pivalyl,acryloxy pivalate.
 9. The method of claim 6, wherein the coatingmaterial is bis-(acryloxyethyl) hexahydrophthalate.
 10. The method ofclaim 6 wherein the coating material is bis-(acryloxyethyl) phthalate.